This invention relates generally to television (TV) signal switching and particularly to an automatic switching system that selectively supplies television signals to a television signal utilization device, such as a television receiver or a video cassette recorder (VCR), either from a decoder output or from a cable (antenna) input in response to operation of the decoder. In conventional cable antenna television (CATV) systems, a television receiver is typically connected to an RF output terminal of a decoder. The decoder, which has an RF input terminal for receiving a plurality of program channel signals from a CATV distribution system, produces a signal output on only one channel. This channel is generally VHF channel 3 or VHF channel 4, referred to herein as channel 3/4. This arrangement restricts the use of the television receiver to channel 3/4 and requires that program channel selections be made via the tuner that is resident in the decoder. The variety of controllable functions generally available on a modern television receiver are therefore not available to a viewer. It is also unfortunate since in most CATV systems, the great majority of the program channel signals are not encoded and are capable of being received and processed directly by a television receiver that has a tuner capable of receiving the program channel frequencies, i.e. a "cable ready" television receiver. Viewers often use a signal splitter, in conjunction with a manually operable A/B switch, to permit coupling of either the decoder output signal or the CATV input signal to the television receiver. This arrangement however requires manual operation of the A/B switch and introduces signal loss (approximately 3 dB) because of the signal splitter.
In U.S. Pat. No. 4,783,846 issued 11/8/88, a number of different television signal switching arrangements are shown. The patented circuit couples a cable input signal through a self-contained switch device that includes a signal splitter that has a first output that goes to the converter input and a second output that goes to a stationary relay contact. The other relay contact may be supplied from the converter output and the movable relay contact is connected to an RF device such as a television receiver. The relay is energized from the converter by means of power supplied over the coaxial cable that interconnects the switch, the converter and the television receiver.
In the above cross referenced copending United States patent application, a simple add-on switch arrangement is provided that is powered from an external power wire. The external power wire is plugged into a switched AC outlet on the decoder and it is therefore energized whenever the decoder is energized. That circuit also has a signal splitter however.
The circuit of the present invention differs significantly from that of the patent and the copending application in that the need for a splitter is eliminated. In normal mode, practically no loss is encountered in the signal supplied to the decoder. In bypass mode, the input television signal is directly routed to the utilization device with only a small amount of signal being diverted to the decoder system. In most CATV systems, the decoder is tuned to a "homing" channel even when the decoder is nominally turned off. Diversion of some signal in the bypass mode enables the decoder to receive data from the head-end. The data is in digital form. Consequently, its signal to noise ratio may be much lower than that of video signals. The small amount of signal provided to the decoder system in the bypass mode keeps the decoder in communication with the cable head-end when the input signal is being supplied directly to the utilization device. Accordingly, with the present invention, the viewable signal is not diminished by a splitter and the decoder is kept in communication with the cable head-end.